1. Field of the Invention
The present invention generally relates to a system for transmitting images through transmitting media, for example, optical fibers, without digitizing images (hereafter called "direct image transmission") and, more particularly, to a system for direct image transmission by spectral coding.
The spectral coding method includes changing a portion of the spectrum of light into two-dimensional form according to the size of an image using the parallel processing property of the wavelength of the light wave, so that each pixel in the two-dimensional image corresponds one-to-one with light having a specific wavelength.
2. Description of the Related Art
Conventional transmission of information through optical fibers has been accomplished by following steps: at the transmitter, digital signals acquired by digitizing one-dimensional or two-dimensional information are converted into the corresponding light pulse train by optical encoding, input to optical fibers and transmitted; at the receiver, the transmitted light pulses are converted into desired types of signals using optical and electronic devices. However, such conventional continuous transmission of information using pulses cannot be applied to the high speed transmission of the information of large capacity such as HDTV (high definition television) images or three-dimensional images.
New transmission methods have been suggested for direct transmission of light which forming images through a transmission medium, for example, optical fibers, to overcome the above problem. According to such direct image transmission methods, transmission time can be reduced and the transmission system can be simplified since there is no need for conversion of electrical signals to optical signals and for coding at the transmitter, and no need for conversion of the received optical signals to electrical signals and for coding at the receiver. Furthermore, this direct image transmission method permits high speed transmission of larger volumes information compared to the conventional transmission methods because it transmits optical information in parallel using the parallel characteristics of light. Thus, direct image transmission technology will be very important for next generation communication technologies.
Image transmission methods using spectral coding include a method of varying the wavelengths of dye lasers by tuning them to the positions of pixels [see Applied Optics Vol. 22, No. 13, 1983, pp. 2029-2033], a method of matching light from a xenon arc lamp to a linear portion of an image by grating scanning [see Applied Optics Vol. 22, No. 23, 1983, pp. 3826-3832], and a method of symmetrizing a spectrum which is spread into one direction by fixed gratings, to a linear portion on an image by lens. However, such methods need electrical drive equipment in order to transmit an entire picture since it is necessary to scan in one or two directions in order to transmit two-dimensional images, or tune image or grating simultaneously. Thus such methods have a problem that the transmission velocity thereof is rather lower than digitized transmission method.
Recently, instead of using scanning devices, a method of using an array comprising a plurality of laser diodes (LD) having a specific wavelength in two dimensions and assigning the wavelength of each LD to each pixel has been suggested. [see Optics Letters Vol. 17, No. 8, 1992, pp. 613-615]. This method, however, is basically inappropriate to image transmission requiring high resolution since there are many problems in producing LDs having wavelength differences for the required high resolution and adjusting the spacing of the LD array.